With the increasing importance of indoor air cleaning, various methods for simultaneously removing particles and gaseous substances present in the room have been developed. These air cleaning techniques include filter type, electrostatic precipitation type, plasma type, UV/photocatalytic type, hybrid type with several methods, etc.
Among others, it is known that the air cleaning method using plasma has a significant effect on the removal of contaminants. Electrons and radicals generated by a plasma discharge phenomenon remove most harmful gases such as volatile organic compounds (VOCs), NOx, CFCs, etc. due to high oxidation and exhibit excellent sterilizing effect, and oxygen anions combine with pollen, fine dust, etc. that cause allergic symptoms and allow these particles to agglomerate together to be easily removed.
The plasma discharge can be divided into corona discharge and dielectric barrier discharge.
Corona Discharge
A corona discharge electrode comprises a pointed cathode and a flat counter electrode. When a negative high voltage is applied to the cathode, electrons emitted from the cathode collide with particles to generate cations, and the generated cations are accelerated toward the cathode by electrical attraction and collide with the cathode to emit high-energy secondary electrons. These high-energy electrons and heavy particles cause an inelastic collision to generate chemically reactive species. FIG. 1 shows a typical structure of a corona discharge electrode, in which (a) shows a single needle electrode type and (b) shows a multiple needle electrode type.
The corona discharge electrode is easy to manufacture and has a simple structure, resulting in low prices. However, a large amount of ozone is generated during the discharge and its long lifespan does harm to the human body. Moreover, the generated anions have a very short lifespan, and the amount of ultraviolet rays produced is also small, resulting in a low sterilizing effect.
Furthermore, the plasma volume is very small, and thus the plasma treatment area is limited to a small area. Accordingly, the number of cathodes has been increased to increase the treatment area, but even in this case, a micro arc (streamer) is generated in a direction perpendicular to the electrode gap, and this streamer is generally focused on the same spot, localizing the treatment effect.
To avoid these problems, a dielectric barrier discharge has been proposed.
Dielectric Barrier Discharge (DBD)
A dielectric barrier can generate a high-power discharge at atmospheric pressure, does not require a complicated pulse power supply, and thus is widely used in the industry, particularly for ozone generation, CO2 laser, UV source, pollution treatment, etc.
FIG. 2 shows a typical structure of a dielectric barrier plasma electrode. As show in FIG. 2, a dielectric barrier discharge (DBD) apparatus comprises two parallel metal electrodes. At least one electrode is coated with a dielectric layer. When an insulator is used, in the case of DC power, the flow of current through the electrode is impossible, and thus an AC power is used to generate the plasma. For stable plasma generation, the gap between the electrodes is limited to several millimeters, and a plasma gas flows through the gap.
The dielectric barrier discharge is also called a “silent discharge” because there is no discharge that locally causes a pulse or noise. The discharge is ignited by a sine function or pulse-type power. Depending on the composition of a working gas and the voltage and frequency, the discharge is a filament-type or glow-type discharge. The filament-type discharge is formed with a micro discharge or streamer that develops on the surface of the dielectric layer.
Here, the dielectric layer serves to block a reverse current and avoid a transition into an arc, thus enabling the operation in a continuous pulse mode. Moreover, the electrons are accumulated on the dielectric surface, and the streamers are distributed randomly on the surface, thus inducing a uniform discharge.
The dielectric barrier discharge (DBD) has several variations as follows:
Surface Discharge
As shown in FIG. 3, a metal electrode such as silver is provided on the surface of a ceramic plate, and a plate-type counter electrode is provided inside the ceramic plate. Then, when an AC current is applied between the two electrodes, a glow discharge is generated around the stripe-shaped electrode on the ceramic plate. This discharge is distinguished from the silent discharge, which will be described later, due to the generation of noise during the discharge. This method is effective for the generation of ozone, and a related prior art includes Korean Patent No. 10-0747178.
Silent Discharge (Volume Discharge)
The silent discharge is a typical structure of the dielectric barrier plasma electrode, in which an insulator such as glass is put on one or both electrodes parallel to each other with a gap of several millimeters (mm), and when an AC voltage is applied thereto, small pulse discharges occur in countless numbers without causing the glow discharge. This is called the silent discharge and is widely used in the industrial fields such as the removal of harmful gases due to the generation of active ions.
FIG. 4(a) shows a plate-type dielectric barrier electrode structure. According to this structure, the electric field applied to the surface is uniform, and thus charges are non-uniformly accumulated in the dielectric with a specific statistical distribution pattern, which induces a streamer discharge, not the glow discharge, thus reducing the amount of ultraviolet rays produced.
FIG. 4(b) shows a mesh-type DBD structure, a variation of the plate-type DBD. According to this structure, a mesh electrode is used instead of a typical plate electrode such that the concentration of electrons in the plasma is uniformly distributed due to electric field enhancement in a reactor as well as the geometric structure of the mesh electrode, unlike the typical streamer discharge, thus generating a multi-glow discharge with excellent uniformity and efficiency of plasma. As a result, compared to the existing corona discharge and typical DBD discharge, it is possible to generate a plasma with a large amount of ultraviolet rays and a large amount of active species such as OH radicals, atomic oxygen (O), etc. However, this structure tends to generate noise and exerts high counter-pressure against the flow of fluid due to a high discharge voltage and a narrow gap between the electrodes. Accordingly, as disclosed in Korean Patent Publication No. 10-2002-0046093, it is necessary to extend the electrodes having the same structure in parallel to increase the processing capacity, but the structure is complicated, and the generation of counter-pressure cannot be avoided due to the cross-sectional area of the electrodes.
As a method for solving the problem of the generation of the counter-pressure, Korean Patent Publication No. 10-2009-0097340 discloses a method of forming a through hole that penetrates an electrode. This through hole is not a specific structure that is used only in this publication, but is disclosed in various documents and is widely used to avoid the counter-pressure. Moreover, a method of forming a gap between two electrodes used in this publication employs macroscopic units in millimeters (mm) or more by the structural design of the mechanism, which corresponds to the typical method, not a micro gap method, and this method has various problems such as requiring a high voltage as an applied voltage.
FIG. 4(c) shows another electrode structure, called a micro cap discharge, which generates a strong plasma using a very small discharge gap between electrodes, which is several tens to hundreds of micrometers. This method generates a loud noise and a large amount of ozone during discharge, and thus it is necessary to control the applied voltage so as not to generate the streamer. Moreover, the probability of contact between air and active species in the plasma section is much higher than other structures, and thus large amounts of species effective for the air cleaning and sterilization are generated, thus providing a good sterilizing effect and generating less noise and ozone, compared to the mesh-type DBD. A related prior art includes Korean Patent Publication No. 10-2006-0017191.
However, according to this method, it is necessary to form a micro gap between the electrodes, which complicates the structure, and there is a method of supporting the structure with an insulator from the outside of the metal electrode to implement the gap.
Moreover, in the case of Japanese Patent No. 2009-78266, a through hole is used in an electrode to facilitate the flow of fluid, and an insulating spacer (a spacing device) for forming a gap between electrodes is inserted into the electrodes. However, in the case of the above method, in order to form the spacer, a ceramic insulator, on the electrode, it is necessary to form a dielectric layer on the electrode, form a pattern for the insulating spacer thereof, and then form an insulating layer thereon, and thus there are problems that the process is complicated, the control of the height of the spacer is significantly difficult to achieve, and the production cost is significantly increased.
Similarly, in the case of Korean Patent Publication Nos. 10-2012-006402 and 10-2012-0065224, the use of the through hole is the same as above and, in the case of the formation of the gap between the electrodes, a natural surface unevenness occurring during the formation of the dielectric is used as a gap, without forming the gap using a spacer. However, in the case of this surface unevenness, the shape is random, and thus the gap between the electrodes is different for each position, which makes it possible to uniformly control the electrode properties, and a large amount of ozone is generated, which are very problematic.
Meanwhile, an electrode structure, in which pallets or beads having dielectric properties are filled in a tubular reactor or the fillers are coated with a catalyst, is also used. However, according to these methods, a loss in pressure occurs due to the dielectric filled in the reactor, and when particulate substances are present in exhaust gas, the reactor may be easily blocked. Moreover, in order to process a large amount of exhaust gas, it is necessary to enclose several tubular reactors in a bundle or collectively, and thus the size of the processing system is excessively increased. Related prior arts include U.S. Pat. No. 5,236,627, U.S. Pat. No. 5,236,672, U.S. Pat. No. 4,954,320, U.S. Pat. No. 5,843,288, and Korean Patent Publication No: 10-2009-0086761.
Underwater Plasma Discharge
Underwater discharge can be used to remove bacteria and viruses contained in water by forming microbubbles in water and introducing gas having a strong sterilizing power, such as hydroxyl group (OH), active oxygen (O—, O2, O3), and hydrogen peroxide (H2O2), in water, and its applications include home appliances, such as washing machine, air conditioners, air cleaners, and humidifiers, food processing or catering services, livestock industry, hospital services, etc. which require sterilization/disinfection solutions.
This method of generating active oxygen and ozone bubbles by the underwater discharge is based on a bubble mechanism theory in which a plasma electrode is located in water and a discharge phenomenon occurs in microbubbles generated when water is vaporized by the discharge heat or introduced from the outside, thus generating radicals such as hydroxyl group, active oxygen, hydrogen peroxide, etc. These radicals oxidize heavy metals contained in water and, at the same time, sterilize bacteria and viruses contained in water.
The dielectric barrier electrode is also mainly used as the plasma electrode used in the underwater discharge, like the air cleaning electrode, and this electrode still remains in the above-mentioned plasma electrode structure. Related prior arts include Korean Patent No. 10-0924649 and Korean Patent Publication No. 10-2009-009675.